1. Field of the Invention
The present invention relates to a current-excitation light emitting element. In addition, the present invention relates to a light emitting device and an electronic device each having the light emitting element.
2. Description of the Related Art
In recent years, a light emitting element using a light emitting organic compound has been actively researched and developed. A basic structure of this light emitting element is formed by interposing a layer containing a light emitting organic compound between a pair of electrodes. By application of a voltage to this element, electrons and holes are separately injected from the pair of electrodes into the layer containing a light emitting organic compound, and a current flows. Then, recombination of these carriers (the electrons and holes) causes the light emitting organic compound to be in an excited state and to emit light when the excited state returns to a ground state. Owing to such a mechanism, the light emitting element as described above is referred to as a current-excitation light emitting element.
It is to be noted that an excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
A great advantage of such a light emitting element is that the light emitting element can be manufactured to be thin and lightweight because the light emitting element is formed of an organic thin film with a thickness of, for example, approximately 0.1 μm. In addition, extremely high response speed is another advantage, because time between carrier injection and light emission is approximately 1 μsec or less. These characteristics are considered suitable for a flat panel display element.
Such a light emitting element is formed into a film shape. Thus, plane emission can be easily obtained by formation of a large-area element. This characteristic is hard to be obtained in a point light source typified by an incandescent lamp or an LED, or a line light source typified by a fluorescent lamp. Therefore, the above-described light emitting element also has a high utility value as a plane light source which is applicable to lighting or the like.
For the light emitting element as described above, in general, at least one of the pair of electrodes is formed using a light-transmitting material and the other is formed using various materials. Light emission from a light emitting substance passes through the electrode formed using a light-transmitting material and is extracted to external.
However, when a material with high reflectivity is used for the other electrode (the electrode which is not formed using a light-transmitting material), there has been a problem in that the electrode formed using the material with high reflectivity also reflects light from an external source, and thus, the contrast is decreased.
In order to solve the problem in that the contrast is decreased, a structure in which a polarizing element, a quarter-wave plate, or the like is provided outside of the light emitting element has been proposed.
However, the use of the polarizing element or the wave plate causes problems such as changes in chromatic characteristics due to wavelength dependence of the wave plate and view-angle dependence. Furthermore, provision of components such as the polarizing element and the wave plate increases the costs and complicates the manufacturing process, which has been a problem as well.